1. Field of the Invention
The present invention relates to RFID couplers and, in particularly, to spatially selective couplers capable of selectively communicating with a targeted transponder from among a group of adjacent transponders.
2. Description of Related Art
Radio frequency identification (RFID) transponders, either active or passive, are typically used with an RFID transceiver or similar device for communicating information back and forth. In order to communicate, the transceiver exposes the transponder to a radio frequency (RF) electromagnetic field or signal. In the case of a passive UHF transponder, the RF electromagnetic field energizes the transponder and thereby prompts the transponder to respond to the transceiver by re-radiating the received signal and modulating it in a well-known technique called backscattering. In the case of an active transponder, the transponder may respond to the electromagnetic field by transmitting an independently powered reply signal to the transceiver.
Problems can occur when interrogating multiple adjacent transponders regardless on whether the transponders are passively or actively powered. For example, an interrogating electromagnetic signal may activate more than one transponder at a given time. This simultaneous activation of multiple transponders may lead to collision errors because each of the multiple transponders may transmit reply signals to the transceiver at the same time.
Several collision management techniques commercially exist for allowing near simultaneous communication between multiple transponders and a single transceiver while reducing communication errors. However, such collision management techniques tend to increase system complexity, cost, and delay response. Furthermore, such techniques are often “blind” in that they cannot locate a given transponder or more specifically recognize the position of a transponder within the interrogating RF electromagnetic field. For example, in a printer-encoder device, the device would not know whether the transceiver was communicating with the transponder proximate to the printhead or not.
Another method of preventing multiple transponder activation is to electro-magnetically isolate transponders from one another. For example, devices or systems may employ an RF-shielded housing or anechoic chamber for shielding the adjacent and non-targeted transponders from the electromagnetic field. In various applications, transponders individually pass though a shielded housing for individualized exposure to an interrogating RF electromagnetic field. Unfortunately, RF-shielded housings add cost and complexity to a system and limit the type (i.e., the size) of transponders they may be processed by the system. Furthermore, many devices are limited with regard to space or weight and, thus, cannot accommodate such shielded housings.
The challenge of avoiding multiple transponder activation may be especially acute in some applications. RFID printer-encoders are one example. RFID printer-encoders are devices capable of encoding and printing on a series or stream of transponders. The close proximity of the transponders to each other, during processing, makes single transponder activation problematic. Moreover, the space, cost, and weight restrictions associated with such devices, among other factors, make collision management techniques or shielding components for alleviating multiple transponder activation less than desirable.
In light of the foregoing it would be desirable to provide a RFID device capable of interrogating individual transponders positioned among multiple adjacent transponders without the need for collision management techniques or shielding components. It also would be desirable if such a device had a relatively compact design to facilitate its use in space-restricted systems, such as mobile or portable RFID printer-encoders.